Riding Lawn Mower Not Cutting Evenly? (Possible Causes & Fixes)

riding lawn mower not cutting evenly

When grass cutting season starts, and you want your yard looking spectacular, problems may sometimes arise. You make a few rows and passes only to find out the grass is uneven. Several things could cause this, and it is something that needs to be tended to immediately.

If mechanical problems occur, always use the process of elimination, starting with the least expensive fix first. It may not be as bad of a problem as you may think. So what are the issues causing the grass to come out uneven, and what are the solutions?

If your riding mower is not cutting evenly, it may be caused to a low tire, an unleveled deck, or broken blades. Otherwise, the barrings may have gone or the mandrel assembly has simply worn out. Check the tires, deck level, blades, pulleys, mandrel, belts, and replace as needed, or simply perform a tune-up. 

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Reasons Why Your Riding Lawn Mower Isn’t Cutting Evenly

1. A Low Tire Can Cause an Uneven Cut

If one of the tires is low, the deck will be unlevel and have an uneven cut. This is the cheapest of the fixes because all it takes is to put the right amount of air pressure. On average, depending on the mower and tires’ size, the front should have 15 psi and the back ten psi.

Different commercial brands will require more. The front will have 25 psi, and the back will have 15 psi. The price of air at gas stations will range from free to $1.50, depending on the location.

If a tire goes flat and needs a plug or patch, it could be as much as $25. Sometimes people want tubes inside the tire, and that costs about $40 to $60. Tubes will keep the tire from going flat in many situations, so it lasts longer.

Sometimes you may find yourself needing a brand new tire. If the tire is damaged on the side, it is most likely unrepairable. Lawnmower tires range from $15 to $150, depending on the retailer and quality and name-brand tire.

2. The Deck May Need Leveling

If the tires have all the proper psi, then move on to the deck. You will need the following to measure the deck height for the perfect cut yard.

  1. A level surface
  2. Wrench set
  3. Ruler
  4. Work gloves
  5. Safety glasses

It takes patience because one side will be too high or too low as you level the other side. The good news is, it does not cost anything to level the deck, only time. There are two easy steps to complete this process.

How Level Mower Deck

Step 1: Level the Deck Side to Side

It would be best if you always leveled a lawnmower deck from side to side first. Put the blades even with each other side by side. The measurement should be within 1/8″ from the blade tips.

The lift link adjustment should turn counterclockwise to lower the deck and clockwise to raise it. Every complete turn moves the side of the deck by 3/16 inches. It will take measuring and turning quite often to put the blade heights within 1/8 inch apart.

Step 2: Level the Deck From the Front to the Back

Move on to turn the blades from the front to the back. Measure the blades. The front should be 1/8 to 1/2 inch lower than the backside. Loosen the jam nut and turn the front link adjustment counterclockwise to lower and clockwise to raise the deck.

Use the wrench to loosen and tighten the jam nut. It is critical to tighten the jam nut when completed. If this is not done, it can damage the entire deck and possibly cause injuries.

3. The Blades Can Be the Problem with an Unlevel Cut

Blades can go bad when cutting over pine cones, tree branches, or other yard stuff other than grass. If the blade hits a solid object, it can crack or break the blade. When this happens, you will notice the grass will be uneven.

How to Change Mower Blades

It is essential to follow these steps to change the blades. If they are put on the wrong way, the yard will become even more uneven. The mower will have to be lifted with a jack and stands to complete this task.

Step 1: Take Off the Old Blades

The rule of thumb is if one blade needs to be changed, you have to change them all. Blades cost around $12 to $35 per blade. You will need a board to block the blades from turning because as the bolt turns, the blades will turn.

Before you take the blades off, take notice of how the cutting side is placed. That will tell you how to put the new set on.

Step 2: Put the New Blades On the Deck

Put all of the blades on one at a time and screw the bolt on hand tight. Come back with the board and place it to where the blades will not turn. Tighten each bolt with a ratchet and socket to fit the bolt.

It is critical to have the blades the right way and have the bolt as tight as possible. If the bolt is not tight enough, it can throw the blade off the deck. Not only do severe injuries occur, but they can kill someone if the blades fly off.

4. The Barrings May Have Gone Out on the Pulleys

If the barrings go out on the pulleys, you will notice a heavy shaking in the deck. The common cause for the barrings to go out is not enough grease. The spindles, pulleys, tires, and everywhere grease is required, there is a grease zerk.

When replacing the pulleys, the belt needs to be removed first. A diagram is on the deck or in the owner’s manual to show how to remove and install the belt. Pulleys cost about $15 to $20.

There is a nut and a bolt on the top and bottom of the pulley. Remove the nut and bolt and the pulley. Put the new one in place and replace the nut and bolt, then the belt.

5. The Mandrel Assembly May Have Failed

The mandrel assembly is what spins the blade and is attached to the deck. Over time, the wear and tear will cause the mandrel assembly to fail. This will also cause the deck to shake heavily.

How to Replace a Mandrel

A mandrel assembly cost around $20 to $50. It is not too difficult to replace the mandrel assembly, but safety must be first. These are the steps to follow.

Step 1: Safety First

You will need to remove the deck for this job, so put the parking brake first. Remove the spark plug wire from the spark plug. This step should happen with any DIY project dealing with the deck and blades.

Step 2: Remove the Deck

Lower the deck all the way and remove all belts. You will notice the arms connecting the deck to the mower. Remove the pins and clips, then the brackets on both sides.

Release the locking tabs and remove the blade cable from the deck bracket. Then take off the spring from the idler arm. You will then be able to remove the deck from the mower.

Step 3: Take the Mandrel Assembly Off

Use a board to block the blades and remove the pulley that is connected to the mandrel assembly. Turn the deck over and remove the blade. Turn the deck back over again and remove the mounting bolts of the mandrel and take it off.

Step 4: Put Everything Back Together

Put the new mandrel assembly in its place and bolt it back up. Put the woodblock back to keep the blades from turning and install the blade and pulley. Ensure the bolt to the blade is as tight as possible.

Step 5: Put the Deck Back On

Follow Step 2 in reverse, then put the sparkplug wire back on. The last instructions will be the first, and the first will be the last. Once the deck is back on, test the blades to see if the project was a success.

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Other Problems That May Be the Issue

The last two things to troubleshoot are the belt or if the mower needs a tune-up. If the belt is cracked, stretched, or dry rotted, it can cause the deck to shake. Any shaking motions can cause the grass to get cut unevenly.

The lawnmower may need a tune-up as well. It may cause the entire mower to shake or choke up. This can cause problems in the level of cutting.

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